In the semiconductor industry, devices are fabricated by a number of processes to produce precisely-defined structures of an ever-decreasing size. Even the slightest structural defect can ruin a semiconductor device, and so to avoid losses of time and effort, detection of defects is critical before a defective device is mass-produced or further processes are performed on a defective wafer. Various inspection tools may be utilized, including those in which a view of a wafer (or other device or object) is compared to a reference image showing the wafer in a defect-free state. A reference image or images may be used, such as views of portions of the same wafer that (ideally speaking) contain the same structural features, for example.
Many optical inspection tools perform defect detection based on comparing gray levels between images. For instance, an inspection image Ix,y may be obtained for comparison to a reference image Rx,y. Defects may be detected by subtracting the images (Ix,y−Rx,y) and using various algorithms to determine the existence (or non-existence) of defects by evaluating the subtraction image. See, for example, U.S. patent application Ser. No. 10/345,097, filed Jun. 15, 2003, which is hereby incorporated by reference for all purposes herein.
Other comparison formulas have been discussed in varying degrees of detail. For instance, in “Golden Template Comparison” by William A. Silver (Sensors, October 1990), the use of a correction function on the inspection image, along with another function performed on the subtraction image is discussed. U.S. Pat. No. 4,579,455 discusses an inspection system that includes summing neighboring pixels to aid in determining the presence or absence of a defect.
However, there is a need for improvement in various applications to improve the detection of defects while also avoiding or eliminating false alarms. For instance, using dark-field imaging on patterned dies may lead to false alarms when gray level comparisons are used alone. For example, differences in the slopes of edges, wafer layer widths, and/or slight defocusing of the image may cause significant differences between reference and inspection image gray levels even though no defect is present.